1. Field of the Invention
The present invention relates to transducers that convert relatively rapid variations in environmental ambient conditions into intensity modulated light signals and more particularly to transducers which utilize the birefringent properties of elastooptic materials to affect this conversion.
2. Description of the Prior Art
Fiber optic transducers of the prior art generally utilize a membrane, movable with variations of the ambient condition. This membrane may be coupled to at least one of a multiplicity of optical fibers having opposing end faces across a gap, that are movable therewith to effectuate a variation of the coupling of light propagating across the gap between the end faces of optical fibers. One such device is disclosed in U.S. Pat. No. 4,300,813 issued Nov. 17, 1981 to R. L. Gravel and assigned to the assignee of the present invention. This device includes two optical fibers, each cut to have end faces substantially perpendicular to an axis and positioned with a small gap between the end faces of the fibers. One fiber mounted to maintain its end face stationary, while the other is cantilevered and coupled to a membrane movable with variations in ambient pressure, thus effectuating relative movement of the optical fibers and thereby the variation of the coefficient of coupling between the optical fibers.
A fiber optic transducer which provides improved sensitivity over that of the Gravel device is disclosed in U.S. Pat. No. 4,293,188 issued Oct. 6, 1981 to D. H. McMahon and assigned to the assignee of the present invention. As in the Gravel device, the transducer disclosed by McMahon comprises a first optical fiber disposed with its end face stationary while the second optical fiber is coupled to a membrane movable with variations in ambient pressure and disposed so that its free end may be laterally displaced from the axis of the first optical fiber in accordance with the movement of the membrane. The sensitivity of the transducer is enhanced inspite of the use of large core. Large numerical aperture, multimode optical fibers, by equipping the opposed faces of the cooperating fibers with regular arrays of equally spaced opaque, absorptive, or reflective gratings providing intensity or phase modulation of the propagating light energy in proportion to the transverse displacement of one grating with respect to the other.
Another fiber optic transducer of the prior art for converting variations in ambient pressure to intensity modulated optical signals is provided by cutting and polishing the ends of two optical fibers, having equal indices of refraction, at predetermined angles with respect to their axes and positioning the angled end faces in parallel relationship a distance apart that is less than the wavelength of the light propagating within the input fiber, creating a gap between the end faces wherein a material with a second refractive index is contained. The angled end faces are held in position by two plates, through which the fibers extend, and are coupled to pressure sensing elements of the system. Variations in pressure cause the distance between the end faces of the fibers to vary producing, via frustrated internal reflection, variations in the optical signal energy coupled between the optical fibers thus creating an amplitude modulated light beam at the output port of the system.
Though the transducers of the prior art, described above, are capable of converting motions in the order of 10.sup.-10 cm to a measurable optical signal modulation, they are susceptible to damage by vibrations or shocks of large amplitude. This damage is generally to the membranes or other moving parts of the system. Thus, it is desirable to provide fiber optic transducers to sense ambient conditions that operate without the use of relative movements between working parts to achieve a modulation of the optical power throughput.
A transducer of the prior art that is more rugged than the devices above described is disclosed in U.S. patent application No. 248,616 filed Mar. 27, 1981 by D. H. McMahon and assigned to the assignee of the present invention. This device includes an optical fiber through which light is coupled from a source to the focal region of a lens wherefrom it is collimated to be incident to a polarizer with the polarization axis oriented at 45.degree. to the vertical. Forty-five degree polarized light is then incident to an elastooptic material such as glass, lucite, and plexiglass which become birefringent under uniaxial stress conditions. Due to the birefringence caused by a stress applied parallel to one of the axes of the elasto-optic material a phase difference is realized between the vertical and horizontal components of the 45.degree. polarized light emerging from the elasto-optic material that is proportional to the applied stress. The orthogonal components are further processed to derive signals representative of the induced phase and therefore the applied stress. This transducer, however, exhibits temperature and pressure head sensitivity, and as a consequence thereof does not provide optimum performance in ambient pressure and temperature variable environments.